The present invention generally relates to Plastic Ball Gate Arrays (PBGA""s), and more specifically relates to a new PBGA design where a ground plane is provided at the bottom layer.
One type of integrated circuit (IC) package is called a Plastic Ball Gate Array or PBGA. The major component of such a package is a substrate which is comprises of a glass epoxy laminate having a copper traces disposed thereon and having one or more vias drilled therethrough. There are two layer PBGA""s as well as four layer PBGA""s. Generally, two layer PBGA""s have higher cross-talk noise and signal impedance compared to four layer PBGA""s. Therefore, typically when there is a need for superior electrical performance, a four layer PBGA (having power and ground planes) is selected over a two layer PBGA, despite the fact that a four layer PBGA generally costs about 10 to 20 percent more than a two layer PBGA. Hence, there is a need for a two layer PBGA with improved electrical performance, such that the cheaper two layer PBGA can be chosen over the more expensive four layer PBGA in high performance applications.
A general object of an embodiment of the present invention is to provide a two layer PBGA which provides high electrical performance.
Briefly, and in accordance with at least one of the foregoing objects, an embodiment of the present invention provides a two layer Plastic Ball Gate Array which includes a metal ground plane at its bottom layer, preferably extending from the center of the package to its edge. The ground plane provides a return path for the signals and preferably reduces the power impedance as compared to conventional two layer PBGA""s which do not include any such ground plane. The ground plane is preferably a metal plane which is connected to ground and voided around the signal and power balls, via and xe2x80x9cdog bonesxe2x80x9d (i.e., thick traces which connect the vias to the ball pad at the bottom layer). Preferably, the plane is grounded through a metal connection to the ball pad at the center of the package and proximate the edge of the package. Preferably, the ground ball pad at the periphery is spaced apart for optimum grounding.
Preferably, the PBGA is configured such that the ground plane serves effectively the same function as the second layer ground plane in a conventional four layer PBGA. The PBGA provides a cheaper alternative to the generally more expensive four layer PBGA. An advantage of such a two layer PBGA is that it uses effectively the same technology as existing two layer PBGA""s. Such a two layer PBGA preferably provides better cross talk performance (especially for high frequency signaling) as well as better thermal performance by having more metal at the bottom layer of the package.